1. Field of the Invention
The present invention generally relates to automatic gain control (AGC) and, in particular to gain controller and gain control method for a radio receiver.
2. Description of the Related Art
Hereinafter, the meaning and interpretation of a term "gain" includes not only amplification but also attenuation. Therefore, a variable gain circuit means a variable-gain circuit and/or a variable attenuator.
There has been disclosed a radio receiver which is designed to suppress cross modulation due to undesired waves in Japanese Patent examined Publication No.5-335857. The conventional radio receiver is provided with a gain controller which controls the gain of a variable attenuator. The output of the variable attenuator is detected by a detector and the detected signal is demodulated by a demodulator.
The gain control is performed based on the signal level of a detected signal which is to be demodulated and the error detection of a demodulated signal. More specifically, when an error in frame synchronization is detected and the signal level is higher than a predetermined level, the gain controller decreases the gain of the variable attenuator to a low value. In other words, when the frame sync error is detected in the case of the sufficient input signal level, the gain controller decides that the cross modulation due to two or more undesired waves occurs. Therefore, by decreasing the gain of the variable attenuator, the cross modulation can be reduced. Contrarily, when the frame syc error is not detected or when the frame sync error is detected and the signal level is lower than the predetermined level, the gain controller keeps the gain of the variable attenuator to a high value.
Another radio receiver using a conventional AGC circuit has been disclosed in Japanese Patent examined Publication No. 7-212262. The radio receiver is provided with a pair of resonance circuits each connected to a saturable reactor. The respective resonance circuits have a higher resonance frequency and a lower resonance frequency than a desired frequency, which provides variable attenuation. In other words, such a circuit is a kind of variable-frequency notch filter. Therefore, undesired waves causing cross modulation can be selectively eliminated.